In recent years, a motor drive control apparatus which detects a possibility of overheating of an element constituting a circuit and suppresses overheating of the element is disclosed (for example, JP-A-2015-177702).
JP-A-2015-177702 discloses that, in the motor control device, a temperature of the element constituting the circuit is calculated based on a temperature of the circuit, a rotation speed of a motor, and a load of the circuit and a duty ratio corresponding to the rotation speed of the motor is lowered when the temperature of the element is equal to or higher than a predetermined value.
In a fan or the like where a resinous impeller is coupled to a motor, when the impeller which is a non-heat-generating component rotates at a high speed under a high environment temperature where a blade of the impeller is easy to deform, there is a possibility that the blade is deformed and an abnormal states such as damage of the blade and damage of a casing of the fan occurs. In general, an operation guarantee temperature is predetermined for use of a device using such a motor and it is recommended to be used under an environment temperature where the possibility of occurrence of such an abnormal state is low. However, in actuality, it is assumed that there is a possibility that the device is used in a state where an ambient temperature exceeds an operation guarantee temperature, and thus it is desirable that the possibility of occurrence of an abnormal state can be lowered even when the device is used in such a state described above.
In a method of suppressing overheating of an element which generates heat by detecting a temperature of a circuit as described in JP-A-2015-177702, the duty ratio is not limited unless the temperature is considerably high. Therefore, such a method cannot be used to prevent occurrence of an abnormal state such as damage of a non-heat-generating component such as the blade of the impeller as described above.
Further, when an IC or the like which causes thermal shutdown at a temperature at which a temperature of an element is relatively low is used for the motor drive control apparatus, there is a possibility that occurrence of an abnormal state can be prevented, but there is a problem that a situation that a motor does not rotate increases. For example, in a fan, there is a problem that the function thereof is frequently impaired in spite of being used for cooling a heat-generating object.